1. None of the 4 people must get down before you....

2008-05-27T21:45:00.000+05:30

1. None of the 4 people must get down before you. Hence all 4 must get down at 5th/6th/7th/8th floor.

Probability that a person gets down at 5,6,7,8th floor is 1/2

Hence P that he does not get down in 1,2,3,4 is 1/2 (obviously)

Since there are 4 of 'em, the probability that all 4 do not get down in 1,2,3,4 floor is 1/2^4 which is 1/16

Answer: 1/16

2. 1 of the lights will be on because of you pressing 5. So, technically among the remaining 4 guys:

a. 1 of em must again press 5 (or not press, it does not matter anyway)

b. The other 3 must press the remaining 2 lights. This can happen with 2 getting down in floor X and 1 getting down in floor Y wherein X and Y are not 5.

**The probability that 2 guys select the same floor X is 1/8*1/8 and floor X can be selected in 7 ways. Therefore, actual probability is 7/64

** The probability that the other guy selects floor Y is 1/8 and Y can be selected in 6 ways (excluding 5th floor and Xth floor)
Therefore probability that he selects Y is 6/8 or 3/4

** Probability that 4th person will select 5th floor is 1/8

*** Hence combined probability that 2 persons will select floor X, 1 will select floor Y and 1 will select 5th floor is

7/64*3/4*1/8 = 21/2048

Are these answers correct?

I dont think we need that data. I have looked at t...

2008-03-29T14:35:00.000+05:30

I dont think we need that data. I have looked at the problem differently.

If 4 people are to get down between floors 1 and 4, how many ways can that happen... etc etc...

I think you need to give the number of people who ...

2008-03-29T09:12:00.000+05:30

I think you need to give the number of people who work on each floor to give a correct answer to your questions :)

Anyway, good show on reviving the blog!

Comments on Coffee House: Swapnalok Complex

1. None of the 4 people must get down before you....

I dont think we need that data. I have looked at t...

I think you need to give the number of people who ...